1. Field of the Invention
The present invention relates to a magneto-optical recording method for accomplishing overwriting at high speed by means of a demagnetizing field, using a magneto-optical recording medium comprising a single perpendicular magnetic layer which serves as a recording layer.
2. Discussion of Background
In recent years, research and development has been energetically undertaken to provide a magneto-optical recording medium utilizing magneto-optical effects, as an optical recording medium on which repeated rewriting is possible, and generally such a medium has been put to practical use. This magneto-optical recording medium has the advantage of large volume, high density recording, non-contact recording and reproduction, and ease of access, in addition to being looked on as a promising medium for utilization in document data files, video and still-picture files, computer memories, and the like, because rewriting is possible. However, there are many technical problems in providing a magneto-optical recording medium as a recording medium with a performance equivalent to or greater than that of a magnetic disk. One of the main problems relates to overwrite technology.
At the present time, to rewrite data on commercially available magneto-optical recording media, it is first necessary to take the step of erasing the original data. The new data may then be written in. These repeated erasures represent wasted time. Overwrite technology can eliminate this drawback. The overwrite technology which has been proposed up to the present time is divided into two types-a magnetic field modulation method, and a light modulation method such as a multi-beam method, and a two-layer film method as described in H. Iida et al.: J. J. of Appl. Phys., Vol. 28(1989) p. 367.
In the magnetic field modulation method, recording is carried out by reversing the polarity of the applied magnetic field in accordance with the recorded data while the light is maintained at a uniform intensity. In this method, studies have been made with respect to the use of a flotation type magnetic head to reverse the magnetic field at high speed, for instance, as disclosed in Japanese Laid-Open Patent Applications 63-204532 and 63-217548. However, changing the medium is difficult. This can cause problems. Also, only one side of the magnetic-optical recording medium can be used, which cuts the recording capacity in half.
In the light modulation method, on the other hand, recording is performed by an ON/OFF action or intensity modulation of a laser beam to be applied corresponding to a recording data, while applying a uniform magnetic field.
The multi-beam method of the light modulation method is a pseudo-overwrite method in which two or three laser beams are used and data is recorded and erased by reversing the direction of the magnetic field for each rotation of the magneto-optical recording medium. However, this method has the drawback that the structure of the device is complicated so that the cost is therefore increased.
In the two-layer film method, the recording layer of the magneto-optical recording medium is made up of a two-layer film and overwriting is achieved, for example, as disclosed in Japanese Laid-Open Patent Application 62-175948. In the method outlined in this Laid-Open Patent Application, a magneto-optical recording medium provided with, for example, a recording layer composed of TbFe and an auxiliary layer composed of TbFeCo is used. In this method, it is intended to achieve overwriting, after initialization, by applying an external magnetic field and laser beams with different powers. Specifically, in recording by this method, the auxiliary layer is magnetized in one direction by means of a magnetic field of about 4 KOe for initialization prior to recording, and a high-output laser beam is applied so that the temperature of the medium is elevated to a temperature T where T&gt;Tc2 in which Tc2 is the Curie temperature of the auxiliary layer. A recording magnetic field is then applied in the opposite direction to the initialization magnetic field, so that the magnetization direction of the auxiliary layer is reversed, and when the medium is cooled, data is recorded by the transference of this magnetization to a memory layer. In addition, erasure is performed by applying a low-output laser beam so that the temperature of the medium is elevated to a temperature T where Tc1&lt;T&lt;Tc2 is which Tcl is the Curie temperature of the recording layer, and transferring the direction of magnetization of the auxiliary layer to the memory layer. However, while this method has the advantage of high speed, the laser power during write-in is high, and there is the additional problem that an extremely large magnetic field must be applied in the advance initialization process for overwriting. As outlined above, many methods have been proposed as overwrite methods in magneto-optical recording, but all of these have as many demerits as merits, and many breakthroughs are required to put these methods to practical use.
An overwrite method utilizing a demagnetizing field in has been proposed (Han-Ping: Appl. Phys. Lett., 49, p 8 (1986)). In this method, a magneto-optical recording medium provided with a recording layer with a compensation point several tens of degrees (.degree. C.) higher than room temperature is used, and overwriting is possible without using an external magnetic field.
To briefly outline the principles of this method, recording is performed by applying a first laser beam to the position on the recording layer where recording is intended. When this laser beam is applied, the laser-beam applied section is heated to a temperature higher than the compensation point Tcomp thereof, so that the magnetization in that section is reversed, and at the same time a demagnetizing field is produced in the opposite direction. The magnetization is reversed by this demagnetizing field, so that a domain wall is formed. Then, a recording magnetic domain is formed during the cooling process after the completion of the application of the laser beam.
Erasure is performed by applying a second laser beam directly onto the recording magnetic domain. The temperature of the section to which this laser beam is applied is elevated to a temperature higher than the compensation point Tcomp, so that the magnetization throughout the recording magnetic domain is reversed, and at the same time, a demagnetizing field is formed in the opposite direction. As a result of the formation of this demagnetizing field, the magnetization of the center section of the recording magnetic domain is once again reversed and a second domain wall is formed, but the second domain wall is caused to disappear by domain wall displacement. The recording magnetic domain is erased during the cooling process after the completion of the application of the laser beam.
This method is a type of light modulation method, and the recording medium for this method of a type having a single layer structure. This method is attracting attention because, compared with a two-layer recording medium, there is a high possibility of recording with low power, and a magnetic field for initialization is unnecessary.
However, in this method, the previously-recorded signal must first be detected prior to overwriting, and the detected signal must be compared with a new signal to be written in to make a decision as to whether the laser beam is to be applied or not. Therefore, either two laser beams must be provided--one for recording and one for detection--or one laser beam must provide the two functions of recording and detection, necessitating two rotations for one track. In addition, when the laser beam is applied, the magnetic domain in which data was previously recorded must be precisely positioned. Accordingly, in overwriting, the function of reading the signal which was previously recorded, the function of deciding whether or not to apply the laser beam by comparing this signal with the new signal to be recorded, and the function of positioning the laser beam to be directly applied onto the proviously written-in signal must be provided in an apparatus using this overwrite method. The structure of the apparatus therefore becomes complicated. An additional drawback of this method is the fact that it cannot be applied to the mark-length modulation method.
In Japanese Laid-Open Patent Application 1-119941, an overwrite method is disclosed in which a single laser pulse is used in the formation of a magnetic domain, and a plurality of laser pulses narrower than those used during recording is used during erasure of the magnetic domain. In this method, however, if it is tried to form the magnetic domain shown in FIG. 8(b) by applying, for instance, the laser pulse illustrated in FIG. 8(a), the edge portion of the rear end of the magnetic domain is not fixed, and, as shown in FIG. 8(c), a phenomenon by which a cylindrical magnetic domain moves, following the laser beam, or, as shown in FIG. 8(d), a phenomenon by which the shape of the edge portion of the magnetic domain is deformed, is occasionally observed. Controlling this phenomenon by applying a single pulse of a laser is extremely difficult. Accordingly, this method is unsuitable for the mark-length modulation of signals, and mark edge recording.